A known clutch that controls a contact condition between an armature and a pulley is disclosed in JPH3-28527A. According to the clutch disclosed in JPH3-28527A, the armature and a hub is connected with each other by means of a plate spring, and a transfer torque pulsation absorbing mechanism is accommodated inside the hub.
According to a known clutch disclosed in JPS57-107440A, inner clutch parts and a friction plate are arranged on a driving shaft, and engage with each other. The inner clutch parts are provided at the side of an inner circumference of an outer clutch part (corresponding to a pulley of the present disclosure) and the friction plate is provided so as to be in contact with side portions of the inner clutch parts. Disc springs press the friction plate against the side portions of the inner clutch parts from a direction opposite to a surface of the friction plate that faces the inner clutch parts.
A known friction clutch disclosed in JP2001-200860A includes a rotor, a boss attached to a driven shaft, an intermediate member pivotably attached to the boss and an initial operation means. The rotor, which is rotatably supported by the driven shaft and is driven by an external power, includes a friction surface on one side. The intermediate member includes a friction surface that is contactable to and engageable with the friction surface of the rotor. The intermediate member transfers a friction force generated between the friction surface of the intermediate member and the friction surface of the rotor into a pressing force against the rotor. The initial operation means supplies a relatively small force (an initial force) to the intermediate member. By the initial force, the friction surface of the intermediate member is started to be pressed against the friction surface of the rotor.
A known electromagnetic clutch disclosed in JPH8-2423Y includes a solenoid, a pulley, an inner coupling member, an outer coupling member, an armature plate and a friction plate. The solenoid is fixedly attached to an outer circumference of an end portion of a bearing housing. The pulley is rotatably supported via a bearing at an outer circumference of an end portion of a driving shaft of a water pump, the driving shaft being rotatably inserted into the bearing housing. The inner coupling member is rotatably supported via the bearing at the outer circumference of the end portion of the driving shaft. The outer coupling member is fixedly attached to the outer circumference of the end portion of the driving shaft so as to face the inner coupling member via a viscous fluid for serving as a fluid coupling. The armature plate is fixedly attached to the outer coupling member and faces the solenoid via the pulley. The friction plate made of a magnetic material is attached to a side portion of the armature plate, the side portion facing the pulley, so as to face the solenoid via the pulley.
According to the electromagnetic clutches disclosed in JPH3-28527A and in JPH8-2423Y, a clutch-pressing force pressing the clutch for transferring the torque depends on an attraction force of an electromagnetic coil. Therefore, in case that the electromagnetic clutch is used in the water pump, the clutch-pressing force needs to be increased particularly when an engine speed is high. Consequently, a high current is needed for the electromagnetic coil to generate a large attraction force, and thus a size and a cost of the electromagnetic clutch increase. In addition, in a magnetic point of view, the electromagnetic clutch needs to be thick enough to assure thicknesses of the pulley and the armature, both of which constitute a magnetic circuit, and thus a weight of the electromagnetic clutch increases.
The centrifugal clutch disclosed in JPS57-107440A needs no power for operation, however, conditions for the engagement and disengagement between the inner and the friction plate are determined by the number of rotations of an input shaft. It may be therefore difficult to change the operation state of the water pump at an arbitrary timing in case that the centrifugal clutch disclosed in JPS57-107440A is used in the water pump.
According to the friction clutch disclosed in JP2001-200860A, a rotation axis of the electromagnetic coil is positioned on a rotation axis of the driven shaft, and an armature portion (corresponding to an armature of the present disclosure) is attracted when the electromagnetic coil is energized. However, an electromagnetic force generated by the electromagnetic coil works partly on the armature portion because plural armature portions are arranged discretely. Consequently, the friction clutch disclosed in JP2001-200860A may include an inefficient magnetic circuit, and thus the electromagnetic coil and the friction clutch need to be increased in seizes for attracting the armature portions.
A need thus exists for a water pump for a vehicle, which is not susceptible to the drawback mentioned above.